Electrical resistance unit



sept 13, 1938. K. E. RoLLEFsON 2,130,156

ELECTRICAL RESISTANCE UNIT Filed April 18, 1936 l 5 INVENTOR.

Krf/ c//Sa/l Patented Sept. 13, 1938 UNITED STATES ELECTRICAL RESISTANCE UNIT Karl E. Rollefson, Evanston, lll., assignor to The Muter Company, Chicago. Ill., a corporation of Illinois Application April 18, 1936, Serial No. 75,179

1 Claim.

'I'his invention relates to electrical apparatus and the manufacture thereof, generally. More particularly this invention relates to electrical resistance units of the wire wound type enclosed in a Jacket and a method for making same.

An object of this invention is to provide a method for economically making a durable and reliable resistance unit adapted to stand relatively high temperatures.

Another object of this invention is tc provide a resistance unit that will stand humid atmospheres without material change in resistance value thereof and without materially shortening the useful life thereof.

Still another object of this invention is to provide a wire wound resistance unit treated to remove strains produced in the wire during winding and sealed in a metal jacket or housing with an insulating material which will not react with the metals contacting it and also will not crack or deteriorate during the useful life of the resistor.

Other and further objects of this invention will be apparent to those skilled in the art to which this invention relates from the following specification and claim.

In accordance with this invention I provide a method of manufacturing a resistance unit which is adapted to withstand severe operating conditions particularly relatively high temperatures. The resistance units of my invention are constructed with the wire support of asbestos lumber processed with a 100% Bakelite resin wood oil varnish adapted to stand high temperatures.

. This protects the wire from moisture and moist atmospheres which propagate corrosion. The application of varnish according to my invention, also removes strains produced in the resistance wire during winding and thus eliminates anodic and cathodic areas associated with strains.

'Ihe treated resistance winding is provided with terminals at predetemined places or intervals along the length of the core and then placed into a flat bottom U-shaped metal housing. Flat strips of insulating material are placed underneath each terminal to keep it from touching the metal housing. A suitable insulating material in liquid or plastic state is poured into the metal housing around the resistor winding level with the top of the housing, to hold the winding in place and form a bond with the housing.

Further details of this invention will be set forth in the following specification and the drawing in which, briefly, Fig. 1 shows a cross-section of the varnished and wound resistor core; Fig. 2

(Ci. 20L-67) is a perspective view of part of a resistor with terminals attached thereto, Fig. 3 is a sectional view of one end of the resistance winding and Fig. 4 is a sectional view of the encased unit.

Referring to Fig. 1 of the drawing in further detail, reference numeral i thereof designates a core of insulation material upon which the wire 2 of the resistance unit is wound. This core may be made of Bakelite", ber, asbestos, or the like. However, in the resistance unit of my invention I prefer to use asbestos lumber because of its excellent heat resisting qualities. The core is treated with a 100% Bakelite resin wood oil varnish which is also adapted to stand high temperatures exceeding 225 degrees Fahrenheit. This varnish has considerable body and lends itself admirably well to serve as a bed for the resistance winding. This is particularly true after the varnish is baked until the surface thereof feels dry but the body is still plastic.

I'he winding 2 is placed over the first layer of varnish 3 while the latter is in a soft and plastic condition, permitting the wire to sink into the varnish and eliminate the strains caused in the wire where it is bent over the edge of the core. The wire also forms grooves in the varnish which hold the wire securely in place and pevent shortcircuiting of adjacent turns.

After the wire is wound on the core a second coat of varnish is applied thereto by dipping. The application of this second coat softens the undercoat and permits the wire to sink slowly into the latter to remove residual strain in the wire. The second coat is baked to dryness and forms an isulating and protective layer over the complete winding. The terminals 5 are attached to the core i and pressed into the varnish layer I to form a rm and secure contact with the wire winding. Several of these terminals are applied to the winding at predetermined intervals to dir vide the unit into sections, if desired. These terminals may be made in the same manner as the terminals of the resistor disclosed in Patent No. 1,789,150.

Two layers 6 of high grade asbestos insulating sheet are placed between the wire winding and the metal housing l as shown in Fig. 4. I'his insulating sheet is also impregnated with Bakelite resin wood oil varnish and dried.

However, it is not necessary to employ this asbestos insulating sheet over the resistor winding. In fact, I prefer to dispense with it in cases where the insulation 9, that is, the ller in the container l, is suillcient to protect the winding.

'I'he winding unit is placed into the container the metal surface 8.

While I have shown the insulation 1 only under the terminals 5, I may form these strips U-shaped and place them over the bottom and sides of the terminals 5 whereby these terminals are spaced from the sides of the container as well as the bottom. These strips 'l may be made out of material such as Cardolite which consists of a cured condensation product homogeneous in character; however, these strips may be made of paper or cloth treated with a condensation product formed with cashew nut shell oil and an aldehyde or similar condensation product insu lation materials.

'I'he space between the wound resistor element and the container 8 is filled with an insulation compound sold under the name of Oil Stop" or a compound having similar characterics as outlined below.

1. It must be capable of being in a liquid or plastic state initially and hardening to a. solid state within a reasonable time or by suitable processing.

. 2. It must be a good electrical insulator.

3. It must be impervious to moisture' and must be insoluble in water after setting.

4. It must not react with any of the metals or alloys incorporated in these resistors.

5. It must not crack or deteriorate with age.

6. It should form a good bond with the metal housing, terminals, and other materials it may come in contact with.

7. It must be easy and economicalto handl and process.

8. It should be capable of withstanding the operating temperatures of the resistors indefinitely.

9. It should be inexpensive or if fairly expensive by itself it should be capable of being filled with some inert inexpensive material auch as mica powder, powdered asbestos, etc.

10. It must present a good appearance.

Oil Stop" is a compound produced by the reaction between the oil obtained from the shells of cashew nuts and an aldehyde. This compound is in the form of a viscous liquid that may be poured into the space around the resistor. After this liquid is poured into the container it sets and then can be cured by baking in an oven until it is hard. The substance then becomes hard and inert and has a very high insulation resistance. It is also impervious to moisture and will stand the highest operating temperature to which the resistor vwill be subjected. Furthermore, it bonds well to the other parts of the resistor unit without reacting with any of the metals employed.

In the manufacture of the resistance units herein described I prefer to mix some inert material such as ground mica or asbestos flock with the illler employed in the container I, to act as a binder and reduce the amount of the filler compound required. 'I'hese inert materials, if in small particles. will not subtract' from the efficiency of the nller as an electrical and moisture insulator and if the mixture is properly mixed and thoroughly permeated with the liquid illler, .its adhesive properties are sufilcient for this purpose. This composition is heated or baked as described above, to harden it and bond the container and the resistor into an attractive and serviceable unit.

It will be observed that I have described my process and the resistance unit of my invention in detail. However, modifications thereof may be `vmade without departing from the spirit and scope of this invention and therefore I do not desire to limit this invention to the exact details described except insofar as they may be defined by the claim.

Reference is made to my copending application Serial No. 71,772 iled March 30, 1936, for Electrical resistance unit and method of manufacture.

What I claim and desire to secure by Letters Patent is as follows:

A resistance' unit comprising a core of asbestos coated with resin wood .oil varnish, a resistance wire winding of relatively `small diameter wound over said varnished core, a covering of resin wood oil varnish over said resistance wire winding, a channel-like container for said resistance winding, a wrapping of heat resistant sheet insulating material around said varnished resistance winding, a flat block-like member positioned underneath said wrapped resistance winding for supporting said winding in said channel-like container well spaced therefrom and a filler of heat resisting electrical insulation material completely surrounding said resistance winding and bonding the elements of said winding and said container together.

KARL E. ROLLEFSON. 

